May said that the current care was often “inadequate” and pledged a package of concrete help for youngsters in their schools and local communities, including specialist training for teachers in all secondary schools.

Teachers will be trained to better spot the early signs of anxiety, depression and eating disorders, with fresh links to local health professionals.

“No parent should feel helpless when watching their child suffer. No teacher should feel ill-equipped to deal with a troubled pupil,” the PM wrote on HuffPostUK.

“No teenager should have to leave their local area to seek treatment. No child should ever be left to feel like their life is not worth living.”

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One in ten children aged between five and 16 suffer from a diagnosable mental health problem, and the number of girls admitting to self-harm has trebled in recent years, the PM pointed out.

“It is time for government to do more – to join HuffPost and other campaigners in the fight against something that blights so many young lives,” she said.

Among the package of new measures unveiled on Monday are:

- every one of England’s 3,600 secondary schools to receive mental health ‘first aid’ training for teachers in the next two years

- new moves to end ‘inappropriate’ placing of under-18s in hospitals, often hundreds of miles from their home

- an extra £15m for community clinics and ‘crisis cafes’ as an alternative to going to hospital or seeing a GP

- a comprehensive review of workplace discrimination by employers against those suffering from depression and other conditions

- instant ‘digitally-assisted therapy’ as an alternative to waiting weeks for a face-to-face appointment

- moves to scrap the £300 charge imposed by some GPs for a form to prove patients have mental health conditions

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Children with behavioural disorders are four times more likely to be dependent on drugs, six times more likely to die before the age of 30, and 20 times more likely to end up in prison.

“I want to see mental health addressed not just in our hospitals, but in our classrooms and communities,” May said.

“I want to see a focus on prevention as well as treatment, especially since so many adult mental health problems – which one in four of us will suffer from at any one time – begin in childhood.”

In the annual Charity Commission lecture in London, May was set to make mental health a key plank of her domestic policy agenda for a more active government intervention and a “shared society”.

In her HuffPost UK blog, the Prime Minister said that the package of policies was “part of a wider approach to tackle the burning injustices we face in society”.

“I want us to employ the power of government as a force for good to transform the way we deal with mental health problems right across society.”

She admitted that although David Cameron and Health Secretary Jeremy Hunt had promised so-called ‘parity of esteem’ to give equal importance to physical and mental healthcare, “very often the treatment for those with mental illnesses is inadequate”.

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Health Secretary Jeremy Hunt

“Mental health problems are everyone’s problem. As a society we must face up to that fact. And the announcements I am making today will ensure we do just that,” she wrote.

One in four people has a common mental disorder at any one time and the economic and social cost of mental illness is £105bn – similar to the entire annual NHS budget.

Downing Street added that figures show that for those on lower and middle incomes, more than half of mental health problems start by the age of 14 and 75% by 18.

In 2014, mental health conditions affected almost one in five of all working-age people and around one in seven of people in full-time employment. In the workplace 18 million days were lost to sickness absence caused by mental health conditions in 2015 at a cost of around £9bn a year to employers.

Critics claim that not enough money is being spent on frontline services in the NHS or by councils, although the Government says it is currently investing more in mental health than ever before – spending an estimated £11.7bn a year.

Access and waiting standards have also been introduced for both talking therapies and early intervention in psychosis.

But while the government is investing £1.4bn over the course of this Parliament into mental health support for children and young people, both Labour and the Liberal Democrats say funding for under-18s has not been protected.

Former health minister Norman Lamb told HuffPost on Sunday that there had been a “criminal raid” on children’s funding by local commissioning groups set up by former Health Secretary Andrew Lansley.

In her speech, May will say: “This is a historic opportunity to right a wrong, and give people deserving of compassion and support the attention and treatment they deserve.

“For too long mental illness has been something of a hidden injustice in our country, shrouded in a completely unacceptable stigma and dangerously disregarded as a secondary issue to physical health.

“Yet left unaddressed, it destroys lives, it separates people from each other and deepens the divisions within our society. Changing this goes right to the heart of our humanity; to the heart of the kind of country we are, the values we share, the attitudes we hold and our determination to come together and support each other.”

The new support for schools will involve new trials to look at how to strengthen the links between schools and local NHS mental health staff.

There will also be a major thematic review of children and adolescent mental health services across the country, led by the Care Quality Commission, to identify what is working and what it not.

A new Whitehall ‘Green Paper’ on children and young people’s mental health will set out plans to transform services in schools, universities and for families.

The Prime Minister revealed that she has appointed Lord Stevenson, a long-time campaigner for greater understanding and treatment of mental illness, and Paul Farmer CBE, CEO of Mind and Chair of the NHS Mental Health Taskforce, to lead a review of mental health in the workplace.

Existing laws already protect people when mental illness is classed as a disability, but only when the illness persists for a year or more. For many common disorders such as depression, the average length of illness can be much shorter.

Paul Farmer, Chief Executive of Mind, the mental health charity, said: “It’s important to see the Prime Minister talking about mental health and shows how far we have come in bringing the experiences of people with mental health problems up the political agenda. It’s been on the periphery for far too long.

“The proof will be in the difference it makes to the day-to-day experience of the one in four who will experience a mental health problem this year.”

Dr Gary Wannan, BMA community care committee chair and child and adolescent psychiatrist said: “Until we have the guarantee that extra funding will be provided, there are question marks over whether the measures outlined in this speech will have the necessary impact.”